1-Hexene can be produced in high selectivity via ethylene trimerization using homogeneous, single-site chromium catalyst systems, activated by a molar excess of alkyl aluminums such as methyl alumoxane (MAO) and modified methyl alumoxane (MMAO). In an unavoidable side reaction, a small fraction of the converted ethylene forms polyethylene. This polyethylene polymer can take any or all of the following three forms: (i) it can coat surfaces of the reactor and associated piping; (ii) it can flow out of the reactor in solution with the reaction mixture; or (iii) it can flow out of the reactor as a suspended solid in the reaction mixture. In addition, the formation of polymer can continue downstream of the reaction system due to the presence in the reactor effluent of the still-active homogeneous catalyst.
A number of procedures have been developed for dealing with the problems of polymeric by-product formation and the presence of such polymeric by-product as well as active catalyst in ethylene trimerization reactor effluent. For example, U.S. Pat. No. 6,380,451 discloses a method for killing the catalyst after it leaves an ethylene trimerization reactor by contacting the reactor effluent with an alcohol. An excess of alcohol is required, with a 5:1 mole ratio of alcohol to total catalyst metals being preferred. The preferred alcohol is one with a high enough boiling point such that it can be easily separated from the desired hexene product by distillation. This '451 patent also discloses a method for cleaning the polymer and catalyst residues which deposit on the internal surfaces of the reactor. Polymer is removed by periodic washing with the reaction diluent at a temperature 60° C.-70° C. higher than the trimerization reaction temperature. In addition, this patent discloses cyclohexane as the preferred diluent/solvent in the reactor, as a good solvent is preferred to keep the catalyst system in solution.
U.S. Pat. No. 7,157,612 discloses another method for recovering polymeric by-products contained in the effluent of an ethylene trimerization reactor. Precipitation of by-product polymer within the reactor is minimized by operating the reactor temperature high enough to keep polymer in solution, with a preferred temperature being at least 110° C. Upon leaving the reactor, the effluent is contacted with an alcohol to deactivate the catalyst system. The effluent can then either be cooled, in which case some of the polymer precipitates and can be separated by filtration, or kept hot so that polymer stays in solution. In either case, soluble polymer continues to the downstream distillation columns where it is distilled away from the reaction products, diluents, and alcohol. In this manner, the polymer ends up with the catalyst and heavy by-product residues.
Some other trimerization catalyst systems have been developed which permit the ethylene trimerization reaction to be carried out with high selectivity. For example, U.S. Patent Application Publication Nos. 2008/0058486; 2008/0182989; 2008/0188633; 2008/0200626 and 2008/0200743 all disclose the use of catalyst systems which are soluble in light paraffins, such as C3-C6 iso- and normal paraffins, and which exhibit high activities and improved selectivities at very moderate temperatures of 60° C.-80° C. The use of such light solvents and mild reaction temperatures results in at least a portion, if not most or all, of the by-product polymer being formed as an insoluble precipitate. Some of the insoluble polymer precipitates on the surfaces within the reactor and in the outlet piping. The insoluble polymer which does not stick to these surfaces exits the reactor as a suspended solid.
Prior art methods for dealing with the presence of active catalyst and polymer by-products in the trimerization reactor effluent have encountered some problems. For example, the Cr-based trimerization catalysts typically employ an excess of aluminum alkyl activator relative to Cr compounds. In some systems, this excess can be 100:1 molar equivalents of Al to Cr or more, up to 1000:1. The alcohols used to deactivate the catalyst are not selective to the Cr compounds since the alcohol also reacts with the Al compounds. An excess of alcohol over both Cr and Al is therefore required to ensure all active Cr species have reacted. The '451 and '612 patents discussed above, for example, teach use of a 5:1 molar excess of alcohol to total metals. If the Al:Cr ratio is 200:1 and a molar excess of alcohol to total metals of 5:1 is employed, then the molar excess of alcohol to Cr compound is 1000:1. This exceedingly high excess requirement for alcohol is costly, and also requires the addition of a distillation column for recovery of the unreacted alcohol for efficient utilization of the alcohol.
Prior art methods for separating polymer by-product from the reactor effluent can also be cumbersome. Not all of the polymer contained in the reactor effluent can be separated by filtration, even if the effluent is cooled. Some of the polymer is still in solution, which carries through to the distillation columns. As the polymer is concentrated through successive distillation steps to recover reactants, products, and diluents, the polymer can precipitate and foul the column internals and reboilers.
Finally, build-up of polymeric by-product which remains within the oligomerization reactor itself and in associated reactor piping can be troublesome. After polymer by-products like polyethylene have fouled internal reactor surfaces and piping, it may become necessary to shut down the reactor(s) and wash the reactor(s) and piping out with a suitable solvent or wash liquid which can remove the built-up by-products. Shutting down of the reactor(s) for cleaning and maintenance is, of course, economically disadvantageous because production of the desired oligomerization product is interrupted.
In view of the foregoing difficulties which can arise in dealing with catalyst and polymeric by-products found in the reactor effluent from olefin oligomerization processes, it would be advantageous to provide procedures and apparatus configurations for efficiently and cost-effectively treating such reactor effluent to separate catalyst material and polymeric by-products from the rest of the effluent components. Further in view of the difficulties which can arise as a result of polymeric by-product build-up in the reactors themselves, it would be advantageous to provide apparatus and process arrangements which eliminate or minimize the need for complete olefin oligomerization shutdown during reactor maintenance and cleaning. Such procedures, apparatus configurations and processes are embodied in the separation and cleaning techniques and apparatus configurations described herein.